Carbon-13 and deuterium magnetic resonance are being used to study the biosynthesis of three microbial antibiotics, pentalenolactone, ovalicin, and fomannosin. Each of these substances presents a unique biosynthetic problem not readily amenable to traditional techniques of feeding of radioactive precursors and degradation of isolated metabolites. Stable isotope nmr has been used to trace the course of the complex cyclizations, rearrangements, and fragmentations by which these substances are biosynthesized.